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E.M. Rocha, D.A. da Cunha, E.M. Carneiro, M.C. Alves, E.P. Araújo, A.C. Boschero, L.A. Velloso; Influence of Diabetes on Insulin Secretion in the Tear Film of Rat . Invest. Ophthalmol. Vis. Sci. 2003;44(13):2495.
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Purpose:: Our previous work has identified insulin in rat tears and evaluated the influence of systemic glucose on tear insulin levels. To further understand the role of insulin on the ocular surface and the impact of Diabetes Mellitus (DM) on insulin secretion in the tear film, the present study examined: (1) the presence of insulin and glucose transporter-1 (GLUT-1) in lacrimal gland (LG), (2) the time-course of insulin secretion in the tear film of diabetic rats compared to controls and (3) the glucose-, carbachol-, and K+-induced insulin secretion from isolated lacrimal glands of diabetic rats. Methods:: LG protein expression was evaluated in 8-week old Wistar male rats by Western Blot using anti-insulin and anti-GLUT-1 antibodies. DM was induced with streptozotocin (60mg/Kg). Blood and tears were collected from rats fasted for 12 hours and 5, 15, 30 and 60 minutes after intravenous glucose injection (1.0 g/kg of body weight). Isolated lacrimal glands were stimulated with glucose from 2.8 mM to 16.4 mM, carbachol (200 mM) and K+ (40 mM); supernatants were collected after 1h. The samples of diabetic and control rats were analyzed by radioimmunoassay for insulin and by spectrophotometry for protein and glucose levels determination. Results:: Insulin and Glut-1 proteins were identified in rat LG. The insulin levels in the tear film of diabetic rats were fivefold higher than in controls and responded to systemic glucose injection in spite of reduced and unresponsive levels in the blood stream of those rats. In vitro assays demonstrated that higher glucose concentration, carbachol and K+ significantly increased insulin levels from LG samples of both groups. After 7 weeks, despite the absence of pancreatic islets and reduced blood insulin levels in diabetic rats, insulin secretion was still detected in LG. Conclusions:: These findings confirm that insulin secretion in the tear film is influenced by systemic conditions and that lacrimal gland response may be modulated by the endocrine and nervous systems. In addition, the present data indicate that LG may be an insulin storage site for ocular surface maintenance.
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